In a conventional substrate processing apparatus for processing a wafer, the number of particles moving within a processing chamber or a gas exhaust line is measured in order to check a state within the processing chamber or the gas exhaust line. Generally, an ISPM (In Situ Particle Monitor) is used to measure the number of particles. The ISPM may include a laser beam oscillator for irradiating laser beam into the processing chamber or the gas exhaust line; and a photodetector for receiving light scattered from a particle passing through the laser beam (hereinafter, referred to as “particle-scattered light”). The ISPM converts the received scattered light to an electrical signal and measures the number of particles based on the magnitude of the electrical signal.
Meanwhile, in the substrate processing apparatus for processing a wafer by plasma, the plasma is generated in the processing chamber, so that the photodetector receives light emitted from plasma in addition to the particle-scattered light. Accordingly, the light from plasma may be mistakenly detected as the particle-scattered light, which makes it difficult to accurately measure the number of particles.
For this reason, there has been recently developed a particle monitoring method capable of preventing decrease in sensitivity of monitoring particle by calculating a brightness difference between an image acquired by monitoring the inside of a processing chamber and a reference background image corresponding to an operation state of a substrate processing apparatus (see, e.g., Patent Document 1) or a method for monitoring a contamination status within a processing chamber in real time by distinguishing particle-scattered light from light emitted from plasma by separating scattered light into predetermined wavelength components and extracting desired frequency components therefrom (see, e.g., Patent Document 2).
Meanwhile, a high intensity scattered light may be generated from a relatively large foreign substance (particle) attached to a window of a photomultiplier tube or a window of a laser beam oscillator through which laser or scattered light passes, or a relatively high intensity scattered light such as cosmic rays may enter the processing chamber from the outside. In this case, there may be used a method for excluding influence of the relatively high intensity scattered light from the particle number measurement by removing the scattered light having intensity higher than a predetermined level from the received scattered light.
Meanwhile, along with miniaturization of semiconductor devices manufactured from a wafer, a particle size that affects the performance of the semiconductor devices is reduced to several tens of nanometers. Therefore, the number of particles having a size of several tens of nanometers needs to be measured accurately.
Moreover, in order to more accurately check a state within the processing chamber or the gas exhaust line, it is required to accurately measure the number of particles generated by a specific factor among a multiple number of scattering particles.
However, particles having a size of several tens of nanometers may be generated by another factor different from the specific factor. Besides, the particles having a size of several tens of nanometers may be attached to a window of a laser beam oscillator or a window of a photomultiplier tube. That is, the measured number of particles having a size of several tens of nanometers may include the number of particles generated by the specific factor and the number of particles generated by another factor different from the specific factor. Hence, the number of particles generated by another factor different from the specific factor needs to be subtracted from the measured number of particles having a size of several tens of nanometers.
Patent Document 1: Japanese Patent Laid-open Publication No. 2000-155086
Patent Document 1: Japanese Patent Laid-open Publication No. H11-330053
However, in the above-described conventional methods, the scattered light is distinguished based on the intensity thereof, i.e., the size of particles, so that it is difficult to accurately measure the number of particles generated by a specific factor among a multiple number of same-sized particles generated by different factors.